Please contact me any time for PhD thesis projcts. Possible projects comprise numerical simulation of evolution of massive stars, nucleosynthesis, supernovae, gamma-ray bursts, first stars, accreting neutron stars, formation of massive, intermediate-mass and supermassive black holes, binary star evolution and formation of binary black holes, ...
Background:
Stars born with eight solar masses or more run
through all major nuclear burning phases, forming an iron core that
collapses to make a neutron star or a black hole. During the collapse
phase, the outer layers may be ejected in a spectacular explosion
observed as a supernova, or even a more powerful jet-powered gamma-ray
burst. In some rare cases, this terminal explosion may be mediated by
rotation, e.g., through formation of an accretion disk around a black
hole, or by a rapidly rotating, highly magnetised neutron star. On the
other hand, massive stars are know to be born with fairly high initial
rotation rates. The effect of this rotation is important not only at
the end of the star's life, but can also change how the star evolves.
For example, the limiting minimum mass for making supernovae or the
transition from making neutron stars to making black holes can change
significantly.
Project Outline:
The goal of this project is to study the
effect of rotation on the evolution of a massive star in detail. For
this, you will run numerical simulations of stellar evolution
including rotation. You will study how the evolution of the star
changes in detail as the initial rotation rate increases, and how this
changes the final outcomes. You may also study the effect that the
individual rotationally-induced instabilities have on internal mixing
of composition and angular momentum transport, and the effect of mass
loss on the rotation.
Perspective:
Understanding the details of how rotation
impacts the evolution of massive stars remains one of the major
uncertainties in our understanding of the final fates of massive
stars. Recent gravitational wave observations by LIGO are also
sensitive to the spin of the neutron stars and black holes at the time
of merger in a binary star system, and LIGO may be able to see the spin of
the stellar core during a collapse of a close-by supernova.
Project Details:
References
Background:
After the Big Bang it took about 300,000,000 yr before the first stars
would form - now some 13,000,000,000 yr ago. Unfortunately, we can no
longer observe these stars today directly, even with our best
telescopes. But there is still some "fossil" record of them left
behind, preserved in the oldest stars in our galaxy we can observe,
dating back to pre-galactic times. When the first stars exploded as
supernovae, their ashes were dispersed and the next generation of
stars formed, incorporating some of these supernova debris. We can
now measure these abundance patterns in those old stars (in particular
done by astronomers in Australia, including Physics Nobel Prize
Laureate Brian Schmidt, and using telescopes here in Australia and the
largest telescopes in the world in Hawaii and Chile). In fact, we
have a rapidly growing catalogue of them. To some extent, hence, the
abundance patterns are similar to a genetic fingerprint that allows to
identify the parents.
Project Outline:
The goal of this project is to identify the "parents" of these old
stars in our galaxy, i.e., find out about the now "extinct" first
stars in the universe - what their properties where, how they lived
and died - and even how many parents there were, how different or
alike they were. For this we have to create a data base of predicted
abundance patterns of supernovae from the first generation of stars
(picture left/above) to compare with the elements observed in the old
stars. Your work will be help creating the data base and to compare
with the most recent data we have and develop some analysis tools to
allow you do this task.
Perspective:
This work will be part of a larger project to
understand the nature of the first stars. Depending on findings,
extension to this work may lead to a short publication or letter in a
refereed journal. Extension to 3rd year project or honours thesis may
possible, or lay the ground work for such a project in this field.
Project Details:
References
Background:
For millennia mankind has observed stars by eye, later by telescopes -
optical, radio, X-rays, etc., but always if the form of
electromagnetic radiation. Accordingly, we typically record and
classify the evolution of stars as function of their surface
properties, i.e., their surface temperature and their luminosity (the
Hertzsprung-Russell (HR) Diagram). But stars do not just radiate
light, but also emit neutrino radiation. In fact, even the sun emits
about 7% of all its energy in the form of neutrinos. So far, however,
the only stars we have seen as neutrinos are the sun and the supernova
1987 A - detecting neutrinos is very hard, but technology is
improving, so it is good to make predictions what we may see, or
should see, some day. In fact, evolved stars may shine 10,000,000,000
times more brightly in neutrinos at the end of their life as they
shine in visible light - so observing a star in neutrinos tells us
that is very close to death, maybe weeks to hours - and give us an
early warning about an impending supernova. Additionally, neutrinos
not only have an energy spectrum like electromagnetic radiation, but
they also have "flavour" as a "colour".
Project Outline:
The goal of this project is
produce the counterpart of such an HR diagram but for neutrinos and
make graphical representations. How would stellar evolution tracks
look? Maybe including supernovae? How would snapshots of different
astronomical objects like star clusters or galaxies look?
Perspective:
This work will be part of a larger project to
understand the nature of the first stars. Depending on findings,
extension to this work may lead to a short publication or letter in a
refereed journal. Extension to 3rd year project or honours thesis may
possible, or lay the ground work for such a project in this field.
Project Details:
Background:
The lifetime of a star is highly dependent upon its initial mass: the sun will shine
for a total of about 10 billion years; for a star 25 times the mass of the sun this
reduces to a mere 10 million years. When a star of about ten solar masses or
more reaches the end of its life, it may explode as a supernova and leave behind
a neutron star or black hole. But many stars are not single. Instead, they may
have a close companion star. These are called binary star systems. In some
cases the secondary star has much lower mass than the primary star and hence
way outlive its more massive partner. If the two stars are close enough and the
secondary star expands as it evolves, it may transfer mass to the remnant of the
primary star. The mass then spirals inward toward the star in an accretion disk
emitting X-rays. In case the remnant is a neutron star, the accreted
material accumulates on the surface of the neutron star. When the accreted
layer gets big enough, it can ignite in a bright thermonuclear flash incinerating
the accreted layer of nuclear fuel. We can observe such flashes as Type I X-ray
bursts throughout the entire galaxy. The bursts last just seconds and recur on a
time scale of hours to days. Considering there is some hundred of
such systems active in our galaxy, this makes them the most common
thermonuclear explosion to occur in nature.
Project Outline:
The goal of this project is to model such Type I X-Ray bursts in a
special kind of system: stars in which the companion is a helium white
dwarf stars, hence the accreted material is mostly pure helium. These
are much easier to model than system that accrete material which also
contains hydrogen. You will be using a hydrodynamic code to simulate
such bursts for a variety of conditions, study their behaviour, and
compare to observational data.
Perspective:
This project will get you started on
understanding thermonuclear explosions on neutron stars. An extension
to 3rd year project or honours thesis is possible and welcome.
Depending on progress this work should then lead to a scientific
publication in a refereed journal.
Project Details:
Supervisors:
Alexander Heger (MoCA)
Kais Hamza (Maths)
Mike Bessell (RSAA/ANU)
Background:
After the Big Bang it took only a few minutes
to synthesise the primordial composition of the universe,
essentially only hydrogen and helium, with traces of lithium and
negligible amounts of everything else. All heavier elements were
synthesised in stars. From the Big Bang it would take a few hundred
thousand years before atomic nuclei and electrons combine to neutral
atoms and molecules. And few hundred million years before the first
stars formed. This first generation of stars forged the first heavy
elements in the universe and released them back into outer space
when these stars exploded as supernovae. The material was diluted
with the vast amounts of gas left by the big bang, and then
incorporated into the next generation of stars. This way the
universe became increasingly enriched in heavy elements as we find
them in the crust of the earth, to make up planets, and being
necessary to life. The very first generation of stars is thought to
be quite short-lived, and all of them are gone by now. The second
generation would only have very small trace of the ashes of the
first generation of stars, and being much longer-lived, we can find
them in our galaxy today. The ratio of elements in these ashes
provides important clues as to the nature of the elusive first
generation of stars. But for many of the elements the abundances
are so small that only upper limits can be determined. Yet even
these upper limits provide important clues that we want to use.
Recently the most iron-poor star known was discovered by
Australian Astronomers. Only for a hand full of elements abundances
could actually be measured, while for many other just upper limits
could be estimated. Currently there is, however, no good
statistical model in Astronomy how to best estimate these upper
limits and how to use these upper limits to constrain the nature of
the first stars.
Project Outline:
The goal of this project is to derive
upper limits for abundances of chemical elements and confidence
level for these upper limits provided observational and model data.
Abundances of chemical elements are determined by matching spectral
lines from atomic and hydrodynamical stellar atmosphere calculations
to data taken by the Hubble Space Telescope and by some of the
world's largest telescopes in Chile and on Hawaii. A possible
approach would be to simulate observations given the model data and
the same level of noise, to then determine the detectability and
confidence levels. A second goal is to develop a model to constrain
theoretical data for production of heavy elements by the first
stars.
References:
http://adsabs.harvard.edu/abs/2014Natur.506..463K
http://adsabs.harvard.edu/abs/2015ApJ...806L..16B
Supervisors:
Alexander Heger
Amanda Karakas
Background:
Globular clusters are some of the more
spectacular yet mysterious components of galaxies. They are usually
old objects with some 100,000 old stars. Their exact origin and
formation is not known, however. Whereas it was once assumed they
would form monolithically in one star formation event from just a
single chemically homogeneous gas cloud, modern astronomical
observations allowed us to identify at least two, sometimes more,
distinct stellar populations within most of them, visible in the
colour-magnitude diagram. These different populations are
chemically very similar in some chemical elements, but distinctly
different in others. One of the key differences is in their helium
enrichment. This is what changes the location of the low-mass stars
in the colour-magnitude digram, as observed. But how does it affect
more massive stars, especially those that make supernovae?
Project Outline:
The goal of this project is to model the
evolution massive supernova progenitor stars, of globular cluster
chemical composition, but with varying degrees of helium enrichment.
The project will use a modern stellar evolution code to model
evolution, nucleosynthesis, and supernova explosions of massive
stars. Depending on project progress, an extension could be to
compare to models of rotating massive stars and their
nucleosynthesis. The results will be compared to the observational
data.
Supervisors:
Alexander Heger
John Lattanzio
Paul Cally
Background:
The Kepler satellite mission became famous for finding
hundreds of new planets around stars. To do so, it had to
monitor them in minute detail, including all variations and
oscillations of the star. Amazingly, the data was good enough
to use seismology, similar to what we do on earth to determine
its interior structure, or for the sun ("helioseismology"), to
determine the interior structure and rotation rate of evolved
- old - stars that approach the end of their life. For the
Sun, we can easily observe how fast it rotates on the surface,
and we know that the same rotation rate, on average, is
maintained all the way to the centre. But for all other
stars, the interior rotation rate was not known to date. But
now we have observations. The big questions is whether our
current model for transport of angular momentum and stellar
evolution is good enough to explain this data. Having this
data is a unique new opportunity to test physical models for
the action of rotation inside stars.
Project Outline:
The goal of this project is to model the evolution of a star
like the long past its current age until the Red Giant and
Horizontal Branch evolution phases. The project will use a
modern stellar evolution code, and the code would also be
modified to test different physics models for the action of
magnetic dynamos and hydrodynamic instabilities due to
rotation. The results will be compared to the observational
data.
References:
2005ApJ...626..350H
2002&A...381..923S
Supervisors:
Alexander Heger
Duncan Galloway
Background:
Many stars are not single stars like the sun, but are born as
binary stars, two stars in a close orbit about each other. If
one of the stars is "massive," more than about ten times the
mass of the sun, it may end is life in a supernova and leave
bind a neutron star. In some cases where the other star in
the system is of lower mass, and hence loves longer, the orbit
could be tight enough that as this star evolves it swells up
enough to transfer mass to the neutron star. The accreted mass
accumulates in a layer at the surface, and usually starts some
burning immediately (hot CNO cycle). When the layer gets thick
enough, it may burn in a brief powerful flash burning material
all the way to quite heavy material. This is observed as a Type I
X-ray burst. If the accretion is very slow, however, the
layer may be so cool, the burning does not start immediately,
and when it starts, it may just start hydrogen burning, then
subside. Only after several of these weak flashes, a more
powerful burst might result.
Project Outline:
We will use a hydrodynamic stellar evolution code including an
extended nuclear reaction network to follow the accretion and
burning flashes. The goal is to explore the regime of weak
flashes and where they occur and what is their behaviour as a
function of neutron star properties and accretion rate and
composition (originating from the companion star). A possible
extension of the project is to implement the physics of
gravitational settling in the present code.
References:
2004ApJS..151...75W
2010ApJ...725..309P
2003ApJ...599..419N
2007ApJ...654.1022P
Supervisors:
Alexander Heger
Bernhard Mueller
Anthony Lun
Background:
One of the biggest puzzles in understanding the formation and
structure of Galaxies are the huge black holes in their
centres. Some of them have a billion time the mass of the
sun, even when they are only a tenth of their percent age.
One, highly speculative, theory is that they may start as the
collapse of supermassive stars of maybe a million times the
mass of the sun, from the first, or very early, generation of
stars that precede the first galaxies ("pre-galactic stars").
Whereas supermassive stars of primordial composition either
undergo hydrostatic burning or collapse to black hole, stars
that have some enrichment in material from a previous
generation of stars may instead explode, probably the most
powerful explosions in the universe other than the big bang
itself. But where exactly are the boundaries between
explosion, collapse, and hydrostatic burning?
Project Outline:
The goal of this project is to find the boundaries between
hydrostatic burning, thermonuclear explosion, and collapse to
a black hole for supermassive stars, i.e., stars of some
100,000 times the mass of the sun. You will use a
hydrodynamic stellar evolution code that includes
thermonuclear burning and post-Newtonian corrections for
general relativity for non-rotating stars. The simulations
will start with stars of different initial mass and different
initial composition and will follow the early evolution of
supermassive stars until they either collapse, explode, or
reach hydrostatic burning. A possible extension of the
project is to modify the stellar evolution code to include
post-Newtonian corrections for rotating stars.
References:
2011JPhCS.314a2077M
2012ApJ...749...37M
2001ApJ...552..459H
1986ApJ...307..675F
Supervisors:
Alexander Heger
Bernhard Mueller
Anthony Lun
Background:
One of the biggest puzzles in understanding the
formation and structure of Galaxies are the huge black holes in their
centres. Some of them have a billion time the mass of the sun, even
when they are only a tenth of their percent age. One, highly
speculative, theory is that they may start as the collapse of
supermassive stars of maybe a million times the mass of the sun, from
the first, or very early, generation of stars that precede the first
galaxies ("pre-galactic stars"). Models of supermassive stars of
primordial composition suggest that these either undergo hydrostatic
burning or collapse to a black hole. But these stars do not form at
once, but rather start from a small core that accretes mass at a high
rate, as current simulations of early start formation suggest, until a
super-massive star is built up.
Project Outline:
The goal of this project is to find how
such stars with primordial composition and high accretion rates evolve
and approach the point of collapse to a supermassive black hole, as a
function of this accretion rate. And, in particular, what the mass of
the star is by the time it collapses, i.e., what is the mass of the
black holes formed. For example, is there an upper mass limit, and is
this different from the one obtained for stars with a given fixed
initial mass (see other project).
References:
2011JPhCS.314a2077M
2001ApJ...552..459H
1986ApJ...307..675F
2013ApJ...777...99W
Supervisors:
Alexander Heger
Bernhard Mueller
Background:
Most heavy elements from oxygen to iron are
dominantly made by the deaths of massive stars as supernovae. Whereas
fully understanding such core collapse supernovae requires
multi-dimensional simulations including complicated and expensive
radiation transport physics, there is some progress in developing
simpler approximation formulae for these supernovae given the
structure of the star at the time of its death. Depending on the
explosion properties, supernovae synthesise and eject elements in
different proportions, which can be used as a diagnostic of the
explosion model.
Project Outline:
For this project you will use an analytic
model for supernova explosions and their energies to simulate the
nucleosynthesis of these stars. The result is to be compared to the
abundance patterns - elemental and isotopic - that we find in the in
the universe today, in the sun, and on earth. The goal of the project
is to constrain the properties of the analytic supernovae model in its
ability to reproduce the observed data.
References
2012ARNPS..62..407J
arxiv.org/abs/1409.0540
2002RvMP...74.1015W
2002ApJ...576..323R
Supervisors:
Alexander Heger
Bernhard Mueller
Background:
When a massive star reaches the end if its
life, the core collapses into a neutron star or, possibly, a black
hole. In many cases, at first a shock is launched moving outward,
ejecting the outer layers of the star. But there may not be enough
energy to eject the entire core, or there can be hydrodynamic
interactions in the envelope that push some of the matter onto the
central object. How much of the material falls back will determine
the final mass of the compact remnant that is left behind. If the
mass exceeds the maximum mass for a neutron star, it will collapse to
a black hole.
Project Outline:
For this project you will use an analytic
model for supernova explosions and their energies to simulate the
explosion of these stars. You will then use a one-dimensional
hydrodynamic code modified for proper inner boundary conditions, to
simulate the dynamics of the explosion and how much mass is ejected or
fall back. This will allow you to estimate the remnant mass (some
of the rest mass is carried away by neutrinos). Using a range of
supernova progenitor models, you can make perditions about the
distribution of neutron star and black hole masses.
References
2008ApJ...679..639Z
2012ARNPS..62..407J
arxiv.org/abs/1409.0540
2002RvMP...74.1015W
2003ApJ...591..288H
Supervisors:
Alexander Heger
Duncan Galloway
Yuri Levin
Bernhard Mueller
Background:
Many stars are not single stars like the sun,
but are born as binary stars, two stars in a close orbit about each
other. If one of the stars is "massive," more than about ten times
the mass of the sun, it may end is life in a supernova and leave bind
a neutron star. In some cases where the other star in the system is
of lower mass, and hence loves longer, the orbit could be tight enough
that as this star evolves it swells up enough to transfer mass to the
neutron star. The accreted mass accumulates in a layer at the
surface, in some cases periodically igniting in a flash observed as
Type I X-ray burst. These bursts leave behind ashes that accumulate -
very few things ever escape from the surface of a neutron star. When
the ashes layer gets quite thick, it may burn in a powerful flash
called superburst, fuelled by the carbon left behind in the
ashes.
Project Outline:
You will use a two-dimensional
hydrodynamic code to simulate the thermonuclear runaway and the
explosion of such superbursts. You will follow the onset of nuclear
burning through the thermonuclear runaway to the formation of a shock
wave that travels to the surface of the stars where then a superburst
can be observed. The question to answer is how character of the burst
and the transitions depend on the neutron star properties and of the
accreted layer.
References:
2004ApJS..151...75W
2003ApJ...599..419N
2001ApJS..133..195Z
2012ApJ...752..150K
Supervisors:
Alexander Heger
Aldeida Aleti
Steven Mascaro
Background:
After the Big Bang it took about 300,000,000 yr before the first stars
would form - now some 13,000,000,000 yr ago. Unfortunately, we can no
longer observe these stars today directly, even with our best
telescopes. But there is still some "fossil" record of them left
behind, preserved in the oldest stars in our galaxy we can observe,
dating back to pre-galactic times. When the first stars exploded as
supernovae, their ashes were dispersed and the next generation of
stars formed, incorporating some of these supernova debris. We can
now measure these abundance patterns in those old stars (in particular
done by astronomers in Australia, including Physics Nobel Prize
Laureate Brian Schmidt, and using telescopes here in Australia and the
largest telescopes in the world in Hawaii and Chile). In fact, we
have a rapidly growing catalogue of them. To some extent, hence, the
abundance patterns are similar to a genetic fingerprint that allows to
identify the parents.
Project Outline:
The goal of this project is to identify
the "parents" of these old stars in our galaxy, i.e., find out about
the now "extinct" first stars in the universe - what their properties
where, how they lived and died, and in particular, how many parents
there were, how different or how alike or different they were. You
will be using a data base of predicted abundance patterns of
supernovae from the first generation of stars to compare with the
elements observed in the old stars. A particular goal will be to
assess the likelihood of multiple stars fits as compared to the extra
degrees of freedom introduced. This could be done, e.g., using
metricates like the minimum message length or Bayesian networks. You
may also develop and employ new methods for optimal pattern matching
using as much of the information available from observational data as
possible.
References
Nature,
506, 7489 (2014)
2010ApJ...724..341H
2002ApJ...567..532H
Supervisors:
Alexander Heger
Yuri Levin
Anthony Lun
Background:
One of the biggest puzzles in understanding the formation and
structure of Galaxies are the huge black holes in their
centres. Some of them have a billion time the mass of the
sun, even when they are only a tenth of their percent age.
One, highly speculative, theory is that they may start as the
collapse of supermassive stars of maybe a million times the
mass of the sun, from the first, or very early, generation of
stars that precede the first galaxies ("pre-galactic stars").
Whereas supermassive stars of primordial composition either
undergo hydrostatic burning or collapse to black hole, stars
that have some enrichment in material from a previous
generation of stars may instead explode, probably the most
powerful explosions in the universe other than the big bang
itself. But where exactly are the boundaries between
explosion, collapse, and hydrostatic burning?
Project Outline:
The goal of this project is to find the boundaries between
hydrostatic burning, thermonuclear explosion, and collapse to
a black hole for supermassive stars, i.e., stars of some
100,000 times the mass of the sun. The student will use a
hydrodynamic stellar evolution code that includes
thermonuclear burning and post-Newtonian corrections for
general relativity for non-rotating stars. The simulations
will start with stars of different initial mass and different
initial composition and will follow the early evolution of
supermassive stars until they either collapse, explode, or
reach hydrostatic burning. One possible extension of the
project is to modify the stellar evolution code to include
post-Newtonian corrections for rotating stars; another
extension could be to follow the neutrino signal of collapsing
stars and the neutrino-induced nucleosynthesis in the envelope
of the star, as well as a possible explosion due to the mass
carried away by the neutrinos.
References:
2011JPhCS.314a2077M
2012ApJ...749...37M
2001ApJ...552..459H
1986ApJ...307..675F
Background:
After the Big Bang the universe consisted
basically of just hydrogen and helium; all heavy elements were forged
later, by the first generation of stars. Yes the nucleosynthesis
ashed of this first generation of stars were incorporated in the next
generation of stars and into the first small proto-galaxies and
galaxies that ever formed. Later, the material, possibly re-ejected
from the second generation of stars, though somewhat diluted, gets
ever re-cycled in subsequent generations of stars to the present day.
Observing old and very old stars, some of them having lived almost as
long as the age of the universe, but also some of the more recently
formed ones, in the in our and neighbouring galaxy today and analysing
their composition from their spectra, we can try to reconstruct the
galacto-chemical history of the universe, including its beginning from
seeds for the very first stars.
Project Outline:
To study the chemical evolution of
galaxies and of the universe from its beginning, we need to know stars
produce and eject, in form of winds and supernovae - of a population
of stars - as a function of time. The reason is that both stellar
lifetimes and element production vary with initial mass of the star.
Your task will be to develop a tool that allows to determine the
nucleosynthesis contributions from the first generation of stars for
the use of galacto-chemical evolution models. What is already
available is a library of models of supernovae from the first
generation of stars for varying explosion energies. We will use a
model to determine the best choice of explosion energies and
hydrodynamical instabilities in these supernovae. This model, in
fact, was developed in collaboration with two previous Summer Vacation
Scholars (second reference). If the project goes well, it may be
extended to us the new data in galacto-chemical model codes yourself,
during the Summer Vacation Scholarship or in a follow-up project.
Perspective:
Depending on progress this work may lead to a short publication or
letter in a refereed journal. Extension to 3rd year project or
honours thesis is possible.
Project Details:
Background:
When massive stars of about ten times the mass of the sun or more
reach the end of their life, their centre collapses to a neutron star
or a black hole. At the same time, a supernova shock front may be
launched that that disrupts the stars such that much of the
nucleosynthesis products the star has made throughout its life are
ejected, to finally make new stars or become parts of planets and form
the basis for life. As the shock travels through the star, it also
causes explosive burning in the ejected material. What exactly is
being made and what is ejected, however, strongly depends on the
energy of the explosions. From observations we know that some the
biggest massive stars may explode with tremendous amount of energy,
maybe ten times as much as a "regular" supernova. the prized question
is what exactly is being synthesized and ejected, in order to allow us
reconstructing the history of our galaxy and the chemical evolution of
the universe.
Project Outline:
The goal of this project is to study the
nucleosynthesis of these powerful supernovae, so-called "hypernovae"
using a hydrodynamical code that includes a large nuclear reaction
network.
Your task will be to study the how the nucleosynthesis
products of the star change as the explosion energy is adjusted. The
results can be compared to the ratio of isotopes and elements as we
find them in the milky way, in the sun, and on earth, helping you to
constrain which parameters are physically realistic and necessary to
explain observational data. Simply put, this way you can constrain the
properties of dying stars, simply put, just by looking at the
composition of a piece of rock you may find in your backyard.
Perspective:
Depending on progress this work may lead to a short publication or
letter in a refereed journal. Extension to 3rd year project or
honours thesis is possible.
Project Details:
Background:
Stars more massive than about ten solar masses
run through the full sequence of thermonuclear burning phases to form
an iron core in their centre. Which the core collapses, it is still
surrounded by by layers of increasingly lighter material as one goes
further out. This surrounding material start burning violently as
centre contracts and eventually collapses. This rapid burning causes
violent convection, which in turn causes fluctuations in the density
and velocity field of the infalling matter. These deviations from
spherical symmetry can have significant influence on the dynamics of
the collapse, the supernova mechanism, and likely how big a kick the
neutron star receives when it is born. These fluctuations may play
critical role in our understanding of how supernovae work in the first
place. It may even decide whether a star explodes as a powerful
supernova or collapses to a black hole instead.
Project Outline:
The goal of this project is to study the
these density fluctuations just before the star dies. For this
purpose you will simulate the last few minutes of evolution of star up
to the onset of iron core collapse for a range of massive stars. You
will try to understand the landscape of Mach number and Bulk
Richardson number in supernova progenitors of the material that
determines the supernova properties using estimates based on an
analytic model.
Perspective:
Depending on progress this work will make a
significant contribution to a current work on understanding how
supernovae explode and which stars explode. Depending on progress and
outcome, you work is intended to become part of a resulting
publication. An extension to 3rd year project or honours thesis
is possible.
Project Details:
Background:
When massive stars of about ten times the mass of the sun or more
reach the end of their life, their centre collapses to a neutron star
or a black hole. At the same time, a supernova shock front may be
launched that that disrupts the stars such that much of the
nucleosynthesis products the star has made throughout its life are
ejected, to finally make new stars or become parts of planets and form
the basis for life. As the shock travels through the star, it also
causes some explosive burning in the innermost material that changes
the composition of the material that is actually ejected. The
tremendous neutrino flux emanating from the newly born neutron star
additional contributories by transforming or splitting some of the
nuclei. A huge uncertainty, however, is with how much energy the star
actually explodes, even if we know its entire structure prior to the
supernova explosion. And this, of course, changes which elements the
star forges and how much of each it expels.
Project Outline:
The goal of this project is to study the nucleosynthesis of
supernovae using an analytic model for supernova explosions. This
model makes some assumptions about certain physical parameters of the
explosion. Your task will be to study the how the nucleosynthesis
products of the star change as the model is adjusted. The results can
be compared to the ratio of isotopes and elements as we find them in
the milky way, in the sun, and on earth, helping you to constrain
which parameters are physically realistic. This way you can constrain
the properties of dying stars, simply put, just be looking at the
composition of a piece of rock you may find in your garden.
Project Details:
After the Big Bang it took about 300,000,000 yr before the first stars would form - now some 13,000,000,000 yr ago. Unfortunately, we can no longer observe these stars today directly, even with our best telescopes. But there is still some "fossil" record of them left behind, preserved in the oldest stars in our galaxy we can observe, dating back to pre-galactic times. When the first stars exploded as supernovae, their ashes were dispersed and the next generation of stars formed, incorporating some of these supernova debris. We can now measure these abundance patterns in those old stars (in particular done by astronomers in Australia, including Physics Nobel Prize Laureate Brian Schmidt, and using telescopes here and the largest telescopes in the world in Hawaii and Chile). In fact, we have a rapidly growing catalogue of them. To some extent, hence, the abundance patterns are similar to a genetic fingerprint that allows to identify the parents.
The goal of this project is to identify the "parents" of these old stars in our galaxy, i.e., find out about the now "extinct" first stars in the universe - what their properties where, how they lived and died - and even how many parents there were, how different or alike they were. We want to use a genetic algorithm (an optimization method) to find a match and combination of "ashes" from theoretical models in a large data base containing a wide variety of stellar models and supernova and compare to observational data. The student's task will be to develop a code using a genetic algorithm as optimization method to find out which theoretical data (relative abundances of chemical elements) best matches the our best current observations. Some basic programming experience would be advantageous, some mathematical skills are required, but you definitively need to bring the willingness to learn.
Are you ready to recreate "The Stellar Jurassic Park"?
For some supernova remnants like Cas A we now have detailed observational data that give is distance from the center and velocity w/r the observer. Usually astronomers assume homologous expansion of the ejecta - velocity just scales linearly with distance from the center. Using this relation, various structures in the supernova have been recovered, in 3D, some of them rather surprising, like planar "walls" of material. But are these structures real or just an artifact of the reconstruction procedure? The goal of this project is explore different velocity distributions of the ejecta and how they would appear when reconstructed using the assumption of homologous expansion mentioned above. Assume, for example, some ejecta would come out as a bubble on one side, as we find in supernova simulations. How would such a structure appear? Using and developing 3D visualization as well as data from the literature and publications are essential parts of this project.
OpenCL Sparce Matrix Solver (computer science) |
Implement space matrix solver for CUDA (nVidia graphics card) or OpenCL to accelerate nuclear reaction network solver on current and future computer hardware. | |
Lowest M/Z ONeMg WD (astrophysics) |
Determine the minimum mass for making ONeMg white dwarf stars at the lowest metallicities. How does stellar evolution change for very- and ulta-metal poor stars? | |
Lowest M/Z supernovae (astrophysics) |
Determine the minimum mass for core collapse supernovae at the lowest metallicities. How does stellar evolution change for very- and ulta-metal poor stars? | |
Heavy Metal Snowstorms (theoretical/astrophysics) |
1D simulations of instabilities inside accreting neutron stars due to phase separation. This project may also require some theoretical work, maybe some background in condensed matter physics. | |
Type I X-ray bursts (astrophysics) |
Multi-D simulations of mixing and burning in the thermonuclear runaway of a thin layer of accreted material on neutron stars in a binary star system. | |
Consistent data
mapping (computational fluid dynamics) |
Implement algorithm to conservatively map data from 1D Lagrangian coordinate system to multi-D Eulerian coordinates as initial conditions for numerical simulations. Use multi-D simulations to assess quality of mapping. | |
More than very massive stars
(astrophysics) |
Study the evolution of stars that collapse to black holes beyond the "classical" limit of very massive stars . How does the evolution of stars change in that mass range? | |
Evolution of supermassive stars
(astrophysics) |
Determine the mass limits and dependence on initial conditions, for which supermassive stars collapse, and which still burn, and for how long, before they collpase. | |
Supermassive Supernovae (astrophysics) |
Determine mass limits as a function of metallicity as well as dependence on initial conditions, for which supermassive stars explode. | |
Evolution of the Sun
(astrophysics, thesis/directed research) |
Numerical simulations (1D) of the evolution of the Sun. Try to reproduce the current Sun at the current age, and follow the evolution to late times. How will the sun end it life? | |
AGB stars
(astrophysics) |
Follow the evolution of and intermediate mass stars to late times. Simulate nucleosynthesis in these stars. | |
Supernova remnant masses
(astrophysics) |
Perform numerical simulations of fallback ejecta after a supernova explosion to determine the mass and type of the remnant. Is a neutron star or a black hole formed? | |
IMF of the First Stars
(astrophysics, graduate student) |
Match nucleosynthesis patterns of observed metal-poor stars to nucleosynthesis predictions from stellar models. Try to deduce what stellar masses are the best fit to the observed abunacne patterns. Develop a tool and plotting to also incorporate isotopes. | |
Galactochemical Evolution
(astrophysics, graduate student) |
Determine the evolution of different components of nucleosynthesis products (r-, s-, p-process, etc.) as a function of metallicity. Combine observational data for the evolution of different elements with that of different components for isotopes from nuclear data and nucleosynthesis studies. Construct isotopic galactochemical history of the universe. |
Alexander Heger